Liquid crystal displays (LCDs) are flat panel display devices which depend upon external sources of light for illumination. They may take one of two basic matrix types, intrinsic or extrinsic matrix addressed. The intrinsic matrix type relies upon the threshold properties of the liquid crystal material. The extrinsic, or active matrix (AM), type has an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs), that supplies an electronic switch to each pixel.
In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5-10 .mu.m. The glass sheets must be transparent, and must withstand the chemical conditions to which they are exposed during display processing. Otherwise, the needs of the two matrix types differ.
Intrinsically addressed LCDs are fabricated using thin film deposition techniques at temperatures .ltoreq.350.degree. C., followed by photolithographic patterning. As a result, the substrate requirements therefore are often the same as those for segmented displays. Soda-lime-silica glass with a barrier layer has proven to be adequate for most needs.
A high performance version of intrinsically addressed LCDs, the super twisted nematic (STN) type, has an added requirement of extremely precise flatness for the purpose of holding the gap dimensions uniform. Because of that requirement, soda-lime-silica glass used for those displays must be polished. Alternatively, a precision formed, barium aluminoborosilicate glass, marketed by Corning Incorporated, Corning, N.Y. as Code 7059, may be used without polishing.
Extrinsically addressed LCDs can be further subdivided into two categories; viz., one based on MIM or amorphous silicon (a-Si) devices, and the other based on polycrystalline silicon (poly-Si) devices. The substrate requirements of the MIM or a-Si type are similar to the STN application. Corning Code 7059 sheet glass is the preferred substrate because of its very low sodium content, i.e., less than 0.1% Na.sub.2 O by weight, its dimensional precision, and its commercial availability.
Devices formed from poly-Si, however, are processed at higher temperatures than those that are employed with a-Si TFTs. Substrates capable of use temperatures (taken to be 25.degree. C. below the strain point of the glass) of 600.degree.-800.degree. C. are demanded. The actual temperature required is mandated by the particular process utilized in fabricating the TFTs. Those TFTs with deposited gate dielectrics require 600.degree.-650.degree. C., while those with thermal oxides call for about 800.degree. C.
Both a-Si and poly-Si processes demand precise alignment of successive photolithographic patterns, thereby necessitating that the thermal shrinkage of the substrate be kept low. Those temperatures have mandated the use of glasses exhibiting higher strain points than soda-lime-silica glass and Corning Code 7059 glass in order to avoid thermal deformation of the sheet during processing. As can be appreciated, the lower the strain point, the greater this dimensional change. Thus, there has been considerable research to develop glasses demonstrating high strain points so that thermal deformation is minimized during device processing at temperatures greater than 600.degree. C., and preferably, higher than 650.degree. C.
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four properties which have been deemed mandatory for a glass to exhibit in order to fully satisfy the needs of a substrate for LCDs:
First, the glass must be essentially free of intentionally added alkali metal oxide to avoid the possibility that alkali metal from the substrate can migrate into the transistor matrix;
Second, the glass substrate must be sufficiently chemically durable to withstand the reagents used in the TFT matrix deposition process;
Third, the expansion mismatch between the glass and the silicon present in the TFT array must be maintained at a relatively low level even as processing temperatures for the substrates increase; and
Fourth, the glass must be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.
That last requirement is most difficult to achieve inasmuch as it demands a sheet glass production process capable of producing essentially finished glass sheet. Currently, the overflow downdraw sheet manufacturing process is employed. This process is described in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat. No. 3,682,609 (Dockerty). That process requires a glass exhibiting a very high viscosity at the liquidus temperature plus long term stability, e.g., periods of 30 days, against devitrification at melting and forming temperatures.
Corning Code 7059 glass, supra, is currently employed in the fabrication of LCDs. That glass, consisting essentially, in weight percent, of about 50% SiO.sub.2, 15% B.sub.2 O.sub.3, 10% Al.sub.2 O.sub.3, and 24% BaO, is nominally free of alkali metal oxides, and exhibits a linear coefficient of thermal expansion, CTE, (25.degree.-300.degree. C.) of about 46.times.10.sup.-7 /.degree.C. and a viscosity at the liquidus temperature in excess of 600,000 poises (6.times.10.sup.4 Pas). The high liquidus viscosity of the glass enables it to be drawn into sheet via the overflow downdraw sheet processing technique, but its relatively low strain point (.about.593.degree. C.) is adequate only for processing a-Si devices and not for poly-Si devices.
Accordingly, extensive research has been directed at developing glasses designed to meet at least three general requirements. Initially, the glasses had to be adapted to use in fabricating poly-Si devices. Next, they had to be capable of being formed into sheet by the overflow downdraw process. Finally, they had to have linear CTEs that closely matched silicon.
The fruits of such research are reported, for example, in U.S. Pat. Nos. 4,409,337; 4,824,808; 5,116,787; 5,116,788; and 5,116,789, all issued in the name of W. H. Dumbaugh, Jr. The properties of these glasses, as well as their shortcomings, are reviewed in pending application Ser. No. 08/008,560 filed in the names of Dumbaugh, Jr. et al. and assigned to the assignee of the subject application.
A recent advance in liquid crystal technology termed "chip-on-glass" (COG) has further emphasized the need for the substrate glass to closely match silicon in thermal expansion. Thus, the initial LCD devices did not have their driver chips mounted on the substrate glass. Instead, the silicon chips were mounted remotely and were connected to the LCD substrate circuitry with compliant or flexible wiring. As LCD device technology improved and as the devices became larger, these flexible mountings became unacceptable, both because of cost and of uncertain reliability. This situation led to Tape Automatic Bonding (TAB) of the silicon chips. In that process the silicon chips and electrical connections to the chips were mounted on a carrier tape, that subassembly was mounted directly on the LCD substrate, and thereafter the connection to the LCD circuitry was completed. TAB decreased cost while improving reliability and increasing the permitted density of the conductors to a pitch of approximately 200 .mu.m--all significant factors. COG, however, provides further improvement over TAB with respect to those three factors. Hence, as the size and quality requirements of LCD devices increase, COG is demanded for those devices dependent upon the use of integrated circuit silicon chips. For that reason, the substrate glass must demonstrate a linear coefficient of thermal expansion closely matching that of silicon; i.e., the glass must exhibit a linear coefficient of thermal expansion (0.degree.-300.degree. C.) between 32-46.times.10.sup.-7 /.degree.C., most preferably 32-40.times.10.sup.-7 /.degree.C.
The high viscosity value at the liquidus required for the overflow downdraw process, 600,000 poises (60,000 Pas), has been difficult to obtain in conjunction with the several other properties required for poly-Si devices. Consequently, attention has been given to other sheet-forming processes where the viscosity factor is not of such great significance. These include the float process and a redraw process.
The float process involves drawing a continuous sheet of glass over the surface of a molten metal, such as molten tin. The surface contacting the molten metal is not exposed during drawing, and hence is relatively smooth and free from defects. This has the virtue of requiring finishing of only one surface. It is a primary purpose of the present invention to provide glasses suitable for fabricating panels for poly-Si devices that can be drawn by the float process.